1. Field of the Invention
The present invention relates generally to a bipolar memory system and, more particularly, to a system for driving a bipolar RAM (Random Access Memory).
2. Description of the Prior Art
In general, a bipolar RAM comprises a cell array composed of memory cells each of which includes a flip-flop formed by two transistors cross-coupled as a fundamental unit. Each of the memory cells is connected to a word line (W), a hold line (H) and a pair of bit lines (B, B) by which the memory cells can be selected. In this case, selection of one word line, i.e. raising the potential thereof, is effected by a work decoder composed of row address buffers and word drivers.
One prior art system for driving a bipolar RAM is characterised in that the difference in potential between the selected word line and the non-selected word lines is substantially the same during the read mode and the write mode. However, usually, since the range of potential of the write operation is larger than that of the read operation, the above-mentioned difference in potential is set at the margin of operation of the write operation. As a result, during the read mode, when the state of one word line is changed from the non-selected to the selected mode, in other words, when the potential of one word line is changed from low to high, the time required to charge the word line with stray capacity is long, so that the reading speed of the memory cell is low.
In order to improve the reading speed performance, a system is known in the art for driving a bipolar RAM wherein the state of the selected word line is changed from the read mode to the write mode, whereby the voltage of the selected word line is forced to be higher (Ref. Japanese Public Disclosure No. 50-139635). Therefore, the difference in potential between the selected word line and the non-selected word lines during the read mode is set to be smaller than that during the write mode. As a result, the reading speed of the memory cell increases. However, in this system, as will be explained hereinafter, when the state of the selected word line is changed from the write mode to the read mode, the data written into the selected memory cell belonging to the selected word line may be destroyed.